[802.11] Fix maximum packet length

Previously the maximum packet length was computed using an erroneous
understanding of the role of the MIC field in TKIP-encrypted packets.
The field is actually considered to be part of the MSDU (encrypted and
fragmented data), not the MPDU (container for each encrypted
fragment). As such its size does not contribute to cryptographic
overhead outside the data field's size limitations. The net result is
that the previous maximum packet length value was 4 bytes too long;
fix it to the correct value of 2352.

Signed-off-by: Michael Brown <mcb30@etherboot.org>

src/include/gpxe/ieee80211.h

 /** Maximum cryptographic overhead before encrypted data */
 #define IEEE80211_MAX_CRYPTO_HEADER	8
 
-/** Maximum cryptographic overhead after encrypted data */
-#define IEEE80211_MAX_CRYPTO_TRAILER    12
+/** Maximum cryptographic overhead after encrypted data
+ *
+ * This does not count the MIC in TKIP frames, since that is
+ * considered to be part of the MSDU and thus contributes to the size
+ * of the data field.
+ *
+ * It @e does count the MIC in CCMP frames, which is considered part
+ * of the MPDU (outside the data field).
+ */
+#define IEEE80211_MAX_CRYPTO_TRAILER    8
 
 /** Total maximum cryptographic overhead */
-#define IEEE80211_MAX_CRYPTO_OVERHEAD	20
+#define IEEE80211_MAX_CRYPTO_OVERHEAD	16
 
 /** Bytes of network-layer data that can go into a regular data frame */
 #define IEEE80211_MAX_FRAME_DATA	2296
 
 /** Maximum combined frame length
  *
- * The biggest frame will include 32 frame header bytes, 20 bytes of
+ * The biggest frame will include 32 frame header bytes, 16 bytes of
  * crypto overhead, and 2304 data bytes.
  */
-#define IEEE80211_MAX_FRAME_LEN         2356
+#define IEEE80211_MAX_FRAME_LEN         2352
 
 /** Maximum length of an ESSID */
 #define IEEE80211_MAX_SSID_LEN          32